1. Field of the Invention
The present invention relates to a line radiation preventing element for preventing undesired radio waves from being radiated from lines or wires, and more specifically to a line radiation preventing element for preventing undesired radio waves from being radiated from lines and simultaneously from entering to an electronic appliance from the outside thereof through lines. Here, the lines are power lines, signal input/output lines, control lines, etc. led out of a casing or a chassis of the electronic appliance. Further, the undesired radio waves are induced through the lines inside and/or outside the electronic appliance.
2. Description of the Prior Art
When a cable or a coaxial cable including power lines, signal input/output lines, control lines, etc. is led out of a casing or a chassis of an electronic appliance, since electromagnetic waves (referred to as radio waves, hereinafter) generated inside the casing are superposed upon these lines, the radio waves are transmitted to the outside of the casing through the lines and then radiated to the circumference. In contrast with this, undesired radio waves enter the electronic appliance through the lines as jamming (interfering) radio waves or noise.
The undesired radio waves radiated by one electronic appliance interfere with signals of the other appliances, so that the other appliances may be operated erroneously or subjected to ghost interference, for instance. As a result, the electric stability and the signal quality of the other appliances deteriorate.
Conventionally, in order to remove the interfering radio waves such as undesired radio waves or noise induced by and superposed upon the lines (e.g., the power line, input/output lines, control lines, connecting lines between two circuits, etc.) from the electronic appliance, various elements such as filters, capacitors, choke coils, ferrite beads, etc. have been connected in series to the line upon which interfering radio waves are superposed or between the line and the ground.
In the above-mentioned prior art method, since the line impedance changes to a high impedance value or a low impedance value (shorted) at frequency of the noise superposed upon the line, the interfering radio waves can be reflected from the high impedance element or bypassed to the ground through the low impedance element, with the result that undesired radio waves are prevented from being radiated to the outside or entering into the inside of the electronic appliance.
FIG. 14 shows an example of the prior art reflective interfering wave removing method, in which a first electronic appliance having no choke coil and a second electronic appliance having a choke coil are both connected to a common power source. As shown in FIG. 14, direct interfering radio waves superposed upon a power source line and additionally indirect interfering radio waves reflected from the second electronic appliance both enter the first electronic appliance. Therefore, when the phases of both the direct and indirect interfering radio waves match each other, there exists the case where the first electronic appliance is subjected to a strong interference, as compared when the second electronic appliance is not connected to the common power source. In addition to the above-mentioned radio interference, since the inductance of the choke coil connected to the second electronic appliance is reduced due to the presence of capacitances formed between the choke coil wires, there exists such a problem in that the reflection capability of noise deteriorates markedly in a high frequency range.
In the case of the ferrite beads used for the same object as with the case of the choke coil, there exists no problem with respect to the capacitance between wires, and further noise can be consumed as heat by use of a high ferrite resistance at high frequency. However, since the resistance is several tens ohms at the most, the resistance and the reflection capability are both not sufficient according to the line path. Further, there exists a possibility that the harmful influence of the re-radiation of interfering radio waves as shown in FIG. 14 occurs even in the case of ferrite beads.
Instead of the above-mentioned method such that the load side of the electronic appliance is opened to reflect interfering radio waves, there exists such another method that the load side of the electronic appliance is shorted to reflect interfering radio waves by use of a capacitance connected between the line and the ground. In this method, however, as far as the common power source is used, there arises the same problem as with the case of the choke coil, so that the interfering radio waves are re-radiated. Further, in this method, since the interfering radio waves are shorted to the ground, when circuits are connected to each other and then connected to a common ground line within the same one electronic appliance, there exists a problem in that the mutual interference occurs between these circuit through the common grounded line. In any cases, in the case of the above-mentioned prior art reflective line radiation preventing elements, the same problems as described above arise in the same circuit or between the two circuits of the same appliance or between the two different appliances, with the result that the circuit operation is not stabilized or ringing phenomenon occurs due to the undesired radio wave reflection along the lines within the circuits of the electronic appliance.
On the other hand, in all the electronic appliances and systems, undesired radio waves must be suppressed as small a level as possible. In case leakage or undesired radio waves are radiated from one electronic appliance, there exists a possibility in that the other appliances, facilities, navigation control systems, etc. may be subjected to the harmful influence of the radio interference. In particular, the erroneous operation of medical instruments and automatic control system causes serious results.
In addition, in the case of the recent electronic appliances, the erroneous operation must be prevented perfectly even when signals of extremely small level are processed, in spite of the fact that the size, weight and power thereof are all minimized. Therefore, the anti-noise characteristics or the resistance capability against interfering radio waves has become important more and more. In addition, there exist many chances that the electronic appliances are exposed to interfering radio waves radiated by other appliances or to strong electric fields of broadcasting waves, movable radio instrument waves, etc., the electronic appliance must be protected from these various radio waves.
Recently, the frequency handled by the electronic appliances has increased more and more with the advance of the operating speed of digital elements. Therefore, when the higher harmonics of the fundamental frequency thereof are considered, the frequency of the radio waves having a possibility of being subjected to the harmful influence of undesired radio waves is very high and the frequency range thereof is wide. Further, the interference degree is largely influenced by the gap formed in the casing or the chassis of the electronic appliance and the conditions of the coaxial cable, power supply lines, control lines, etc. led out of the casing or the chassis thereof. The various conditions of generating interfering radio waves as described above are one of the factors which make difficult the countermeasures for the electronic appliances against the interfering radio waves. However, since the power supply lines, signal input/output lines, etc. led to the inside of the electronic appliance must be protected from the interfering radio waves as perfectly as possible, these countermeasures are important.